The recent conversion of a former power plant near AT&T Park into 94 apartments is easy to miss, topping off at 35 feet along the sidewalk and 62 feet alongside the alley that leads into a long dense block.

Yet in its own modest way, the renamed Arc Light building at Townsend Street and Clarence Place is one of the most provocative new works of architecture in San Francisco. It sets an energetic glass box within red brick walls that date back to 1888, a new form snapped onto the old - a juxtaposition that sounds jarring but shows more respect to history than if the original materials and details were mimicked up above.

The result isn't a masterpiece. Economic realities left their mark on a project originally approved in 2006. Still, the transformation is living proof that neighborhoods can evolve in contemporary ways, without losing the character that makes them distinct.

Arc Light also embodies the change reshaping a corner of the city where the South of Market area blurs into South Beach and Mission Bay. As this once-vibrant industrial area declined, the power plant became a warehouse and finally a garage. Then the Giants built their ballpark a block away, condominiums began to sprout, and in 2005 Martin Building Co. purchased the brick relic with an eye to restoring it as housing.

Variations of this saga are seen across SoMa in the form of blue-collar buildings that now contain upscale lofts or social-media firms. What sets Arc Light apart is the form that rebirth takes.

The robust brickwork remains, with a gabled front along Townsend and a two-story masonry wall along Clarence that adds a third level toward the alley's rear. Long-covered openings have been restored but other than that the historic box is as simple as can be.

The change comes 40 feet inside the block; a long container of green glass steps straight up to add five levels of housing behind the masonry. Except for a shallow notch above the brickwork, there's no setback at all, just one form fitted into the other. The addition wears a tight skin, small panes of glass within a dark metal grid, depth added by way of recessed balconies above Clarence where the railings resemble horizonal glass shutters, a sort of visual syncopation.

Invigorating collage

A purist will recoil at the collision in styles. More open-minded passers-by will see something else: an invigorating collage of present and past.

Counterintuitive as this might sound, the abrupt shift from blunt brick to sleek glass allows the historic base to retain its character as a structure unto itself. Approach the building from Second Street, and all that's visible is the jagged gabled front and a hint of the corrugated roof behind it. Only when you're standing across the way does 2012 announce itself.

The balancing act was even more dynamic in the original design by Ian Birchall and Associates that was approved by the Planning Commission in 2006. That scheme treated the addition as two long containers, not one, side by side with a walkway in between. The containers lined up with the structural bays facing Townsend Street, and parted where the gable reached its peak.

But aesthetics were no match for a recession that put the project on hold. Martin revived it by enlarging the addition and, to win funding from the state, increasing the number of affordable units. Birchall was replaced by HKS Architects. Martin Building now bills itself as designer.

Whatever the lineage, today's head-on view is chunkier than the initial version. Another loss is the removal of a small public courtyard that was to sit behind the Townsend facade. The space will instead be filled by a restaurant, although the ceiling's historic wooden trusses remain.

Sign of decade's mood

Those alterations don't undermine the value of the lesson on display.

There's an age-old architectural debate as to the etiquette of adding new space to older structures. "Respect for architectural neighbors means more than the meaningless pleasantry," wrote English architect Hugh Casson in the 1974 anthology "The Future of the Past." "There are occasions for the quick return, the wise-crack, the spirited exchange between individuals."

That's especially true in a setting such as Townsend Street, where too many buildings of the past decade come garbed in red stucco or thin brick. They pay unimaginative homage to the area's blue-collar roots while failing to add anything creative to the mix.

Arc Light is different. The long container of aqua-tinted glass is like nothing else for blocks around. It captures the mood of a decade where our urban culture is very much in flux, and looks to the future as well.